Meiosis Phases

Meiosis is split into meiosis I and meiosis II which are further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps that initiate meiosis are identical in pattern and name to interphase of the mitotic cell cycle. Interphase is split into three phases: Growth 1 (G1) phase: during this very active phase, the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it'll need for growth. In G1, each of the chromosomes consists of one linear molecule of DNA. In the Synthesis (S) phase: each of the cell's chromosomes duplicates to become two identical sister chromatids attached at a centromere and the genetic material is replicated.

History & Discovery

Meiosis was discovered and described for the primary time in echinoderm eggs in 1876 by the German biologist Oscar Hertwig. It was described again in 1883, at the extent of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs.  In 1890 a German biologist August Weismann described the significance of meiosis for copy and inheritance, he noted that if the number of chromosomes had to be maintained then two cell divisions were compulsory to transform one diploid cell into four haploid cells. In 1911, the American geneticist Thomas Hunt Morgan detected crossovers in meiosis within the pomace fly drosophila, which helped to determine that genetic traits are transmitted on chromosomes. The term "meiosis" (originally spelled "miosis") springs from the Greek word μείωσις, meaning 'lessening'. It was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905:We propose to use the terms Meiosis or Meiotic phase to hide the entire series of nuclear changes included within the two divisions that were designated as Heterotypic and Homotype by Flemming.

Meiosis I

Homologous chromosomes are segregated during Meiosis I, which are joined as tetrads (2n, 4c), producing two haploid cells (n chromosomes, 23 in humans) which each contain chromatid pairs (1n, 2c). Because the ploidy is reduced from diploid to haploid, ‘meiosis I’ is mentioned as a reductional division. During Meiosis II the sister chromatids are segregated, creating four haploid daughter cells (1n, 1c), meiosis II is an equational division analogous to mitosis.

Prophase I

Typically the longest phase of meiosis is the prophase I. Homologous chromosomes pair exchange genetic information (homologous recombination) during prophase I. This often results in the chromosomal crossover. This process facilitates pairing between homologous chromosomes and hence accurate segregation of the chromosomes at the first meiosis division. The new combinations of DNA created during crossover are a big source of genetic variation and end in new combinations of alleles, which can be beneficial. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and 4 chromatids, with one chromosome coming from each parent. The process of pairing the homologous chromosomes is called synapsis, non-sister chromatids might get cross-over at points called chiasmata (plural; singular chiasma) during this phase. Prophase I have historically been divided into a series of substages that are named according to the appearance of chromosomes.

Leptotene

The first stage of prophase is the leptotene stage, also referred to as leptonema, from Greek words meaning "thin threads". during this stage of prophase I, individual chromosomes — each consisting of two sister chromatids — become "individualized" to make visible strands within the nucleus.the 2 sister chromatids closely associate and are visually indistinguishable from each other. Lateral elements of the synaptonemal complex assemble during this phase. Leptotene is of very short duration and progressive condensation and coiling of the chromosome, fibers take place.

Did You Know?

Meiosis occurs in eukaryotic life cycles involving amphimixis, consisting of the constant cyclical process of meiosis and fertilization. This takes place alongside normal mitotic cell division. In multicellular organisms, there's an intermediary step between the diploid and haploid transition where the organism grows. Germ cells produce gametes at certain stages of the life cycle. Somatic cells structure the body of the organism and aren't involved in gamete production.

Cycling meiosis and fertilization events produce a series of transitions back and forth between alternating haploid and diploid states. Either during the diploid state (diplontic life cycle), during the haploid state (haplontic life cycle), or both (haplodiplontic life cycle, the organism phase of the life cycle can occur in which there are two distinct organism phases, one during the haploid state and therefore the other during the diploid state). In this sense there are three sorts of life cycles that utilize amphimixis, differentiated by the situation of the organism phase(s).

In the diplontic life cycle (with pre-gametic meiosis), of which humans are a neighborhood, the organism is diploid, grown from a diploid cell called the zygote. The organism's diploid germ-line stem cells undergo meiosis to make haploid gametes that fertilize to make the zygote.